A Retail key and a Volume key are different, yes, but they still activate the same software. If we have a volume key, especially MAK (Multiple Activation Key) which has a longer lifetime, in comparison to the retail key, which most of the time is dead at the time they were shared, volume key is a steal.
Although if you had installed the retail copy of Office, you need a script to add support for volume activation to your copy, which nhanchu has provided you from the mentioned message. Run the script with Administrator CMD prompt and you are done, activated.